Method and Extrusion Line for producing peroxide-cross-linked polyethylene pipes

ABSTRACT

A method and extrusion line for producing peroxide-cross-linked polyethylene pipes in an extrusion line having an extruder, a cross-linking furnace and a flue. The extruded tube is subjected to a stretching process, and the cross-linking furnace has a first heating section that extends from its inlet to an actuated deflection roller and a second heating section from the deflection roller to its outlet. The stretching process in the first heating section and the stretching process in the second heating section are monitored and regulated in such a manner that the necessary stretching of the pipe takes place primarily in the first heating section, and the stretching of the pipe in the second heating section is subjected to zero adjustment. In this case, the height of the deflection roller(s) of the cross-linking furnace and the angle of the heating sections to the horizontal plane can be height-adjustable.

BACKGROUND OF THE INVENTION

The present invention relates to a method of producingperoxide-cross-linked or polymerized polyethylene pipes in an extrusionline that includes an extruder, a cross-linking oven and a withdrawalmechanism or haul-off, wherein the cross-linking oven has a firstheating section that extends from an inlet to an actuated deflectionroller, and a second heating section that extends from the deflectionroller to an outlet, with the extruded pipe being subjected to astretching process. The present invention also relates to an extrusionline having a cross-linking oven comprised of a first heating sectionthat extends from the inlet to an actuated deflection roller, and asecond heating section that extends from the deflection roller to theoutlet.

It is known that the properties of polymers can be improved bycross-linking or polymerization. In contrast to non-cross-linked pipes,peroxide-cross-linked polyethylene pipes have the advantage of enhancedheat resistance, mechanical strength and chemical resistance, whichmakes them suitable for under-floor heating pipes, hot water pipes anddrinking water pipes.

DE 696 22 053 T2 describes an extrusion line for in-line cross-linkingof polyethylene pipes. The extruder of this line has an upwardly angledhead, which opens into a cross-linking oven, in which the pipe issubsequently extruded vertically. The pipe is led along a first heatingsection vertically upward in the cross-linking oven, reversed there by180 degrees around an electric driven deflection roller and then ledalong a second heating section vertically downward, where it isdeflected into the horizontal plane by another roller at the outlet ofthe cross-linking oven and then led through a calibration and coolingunit. On the way through the cross-linking oven, the pipe is led alonginfrared units, which provide the heat into the pipe that is requiredfor cross-linking. The calibration and cooling unit is followed by ahaul-off, with which the pipe is pulled through the extrusion line, andafter which, it is finally coiled.

DE 10 2007 050 939 A1 describes a cross-linking oven, which has a firstheating section that extends from its inlet to an electric drivendeflection roller and a second heating section from the deflectionroller to its outlet. In this case, a horizontally extruded pipe is atfirst deflected by 90 degrees around a roller at the inlet of thecross-linking oven, so as to facilitate its passage along the firstheating section vertically upward in the cross-linking oven, reversedthere by 180 degrees around a deflection roller and then led along asecond heating section vertically downward in the cross-linking oven.The pipe is deflected back into the horizontal plane by a rollerprovided at the outlet of the cross-linking oven.

The extruded melt pipe swells up after its exit from the pipe head ofthe extruder. In order to produce a pipe with a required nominaldiameter, the pipe must therefore be drawn later in the extrusion line.However, this stretching process has disadvantageous consequences forshrinkage, which is a key quality criterion for the final product. Thisshrinkage must be as small as possible.

Therefore, the object of the present invention is to provide a methodand an extrusion line with a cross-linking oven, which guarantee minimumshrinkage of the pipe produced.

SUMMARY OF THE INVENTION

The inventive solution is based on the finding that stretching of thepipe in the first heating section has a smaller influence on shrinkageof the final product than stretching in the second heating section.Therefore, according to the method of the present invention, thestretching processes in the first and second heating sections aremonitored and regulated in such a manner that the necessary stretchingof the pipe takes place primarily in the first heating section and thestretching of the pipe in the second heating section is controlled closeto zero.

Pursuant to advantageous embodiments of the method, the stretchingprocess in the first heating section is monitored by measuring the pipediameters both prior to and after the first heating section, whereas thestretching process in the second heating section is monitored bymeasuring the pipe diameters both prior to and after the second heatingsection. In addition, the rotational speed of the deflection roller maybe controlled in such a way that an optimum stretching process takesplace in the first heating section. Sequence control of the haul-off orwithdrawal speed can take place as a constant function of the rotationalspeed of the deflection roller. This sequence control of the haul-offmay be accompanied by a second control system that reduces the speed ofthe haul-off to such an extent that the stretching process in the secondheating section approaches dose to zero.

The present invention also provides an extrusion line with across-linking oven, which has a first heating section that extends fromits inlet to an electric driven deflection roller and a second heatingsection from the deflection roller to its outlet, wherein the height ofthe deflection roller and the angle of the heating sections to thehorizontal plane are adjustable, and such adjustments are advantageouslylinked.

Such a cross-linking oven is used in pipes with larger diameters andwherein a reversal of 180 degrees on the deflection roller—as isnormally the case in known cross-linking ovens—is difficult toimplement. Through the height adjustment of the deflection roller andthe possibility of adjusting the angle of the heating sections, evenpipes with larger diameters facilitate optimum control during thestretching process in the first heating section, wherein the contactpressure between the deflection roller and the extruded pipe can beadjusted optimally.

The cross-linking oven can also be used for pipes with smallerdiameters, wherein adjustment of the deflection roller and the heatingsections facilitates adjustment of the stretching of the extruded pipein addition to the actuation speed of the deflection roller and thehaul-off speed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described with reference to the following embodiments.In the accompanying figures:

FIG. 1 shows a schematic representation of an extrusion line forproducing peroxide-cross-linked polyethylene pipes in a firstembodiment,

FIG. 2 shows a schematic representation of an extrusion line forproducing peroxide-cross-linked polyethylene pipes in a secondembodiment, and

FIG. 3 shows an enlarged view of the cross-linking oven, which is in thestarting position and is used in the extrusion line in accordance withFIG. 2.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The extrusion line shown in FIG. 1 includes an extruder 1 with a pipehead 2 and a hopper 3. The extruder 1 is supplied with the polyethylenein granular or powder form, stabilizer and peroxide through the hopper 3by means of a dosing system that is not illustrated. This mixture isheated, kneaded and plasticized in the extruder 1. Subsequently, theresulting moldable material is pressed through a continuous annular gapof the pipe head 2. After emerging from the pipe head 2, the hot andstill ductile pipe 4 enters a cross-linking oven 5. The horizontallyextruded pipe 4 is at first deflected by 90 degrees around a roller 6 atthe inlet of the cross-linking oven 5, so as to facilitate its passagealong the first heating section A vertically upward in the cross-linkingoven 5. A larger electric driven deflection roller 7, on which the pipe4 is reversed by 180 degrees, is arranged there and the pipe then runsalong the second heating section B vertically downward in thecross-linking oven 5.

Each of the heating sections A and B has four consecutive infraredradiation modules 8, via which the pipe 4 receives the heat energyrequired for cross-linking.

At the outlet of the cross-linking oven 5, the pipe 4 is deflected by 90degrees to the horizontal plane by another roller 9. After leaving thecross-linking oven 5, the pipe 4 enters a calibration and cooling unit10, where it is cooled down. The calibration and cooling unit 10 in theextrusion line is followed by a haul-off or withdrawal mechanism 11,with which the extruded pipe 4 is pulled through the extrusion line. Theend of the extrusion line is a saw unit 12, with which pipe sections arecut off the extruded pipe 4 to the desired length.

The extruded pipe 4 (melt pipe) swells up after emerging from the pipehead 2, so that stretching of the pipe 4 in the cross-linking oven 5 isrequired to achieve the desired nominal pipe diameter within thetolerance limits to be observed. For capturing measurement data of thisstretching process, the cross-linking oven 5 has four measuring points25 that are illustrated as circles, for example in the form of lasersensors. They facilitate measurement of the diameters before (DO) andafter (D1) the first heating section A, and the diameters before (D2)and after (D3) the second heating section B. The stretching of the pipe4 in the first heating section A is derived from the ratio S1=D0/D1 andthe stretching in the second heating section B from the ratio S2=D2/D3.

The pipe 4, which is led through the cross-linking oven 5, is pulled viathe top deflection roller 7 through the cross-linking oven 5. Here, thespeed, i.e. the rpm, of the deflection roller 7 is regulated as afunction of the mass flow rate at the pipe head 2, in such a manner thatthe stretching required for the nominal pipe diameter to be produced isrealized primarily in the first heating section A.

A line speed that is different from that of the deflection roller 7 canbe set by means of the haul-off 11 located in the extrusion line. If theline speed is greater than the speed of the deflection roller 7, thepipe 4 would be stretched in the second heating section B. This is to beavoided, as this stretching would have a negative influence on thespecified shrinkage to be met. The speed of the haul-off 11 is thereforeadjusted by a sequence control of the speed specified by the deflectionroller 7, i.e. if the roller speed is changed, the haul-off speed alsochanges as a constant function of the roller speed. This sequencecontrol is accompanied by a second control system, with which the speedof the haul-off 11 is reduced to such an extent that the stretching S2approaches close to zero and thus, the expected shrinkage is minimized.

FIGS. 2 and 3 describe another embodiment of the present invention whichdiffers from the previous embodiment only in the fact that a differentcross-linking oven 13 is used in the extrusion line. FIG. 3 shows anenlarged view of this cross-linking oven 13 in its starting position,while the cross-linking oven 13 in FIG. 2 is shown in its operatingposition.

Such a cross-linking oven 13 is used for production of pipes 4 of largerdiameters, e.g. above 32 mm, because reversal of such pipes by 180degrees—as is the case in the cross-linking oven 5 of the precedingembodiment—is difficult to realize in practice.

The cross-linking oven 13 is described in more detail below, wherein thereference signs of the previous embodiment are used for the same oroperationally equivalent components.

The cross-linking oven 13 has a roller 6 at the inlet and a roller 9 atthe outlet, as well as a larger electric driven deflection roller 7arranged between rollers 6 and 9. An infrared radiation module 8 isarranged between the rollers 6 and 7, and 7 and 9 respectively, whereinthe infrared radiation module 8 arranged between the rollers 6 and 7forms the first heating section A, and the infrared radiation module 8arranged between the rollers 7 and 9 forms the second heating section B.

In this embodiment, the diameters DO, D1, D2 and D3 are measured beforeand after the first heating section A and the second heating section B.

The deflection roller 7 forms an axis of rotation 15 with bearing blocks14 arranged on both sides. The two bearing blocks 14 are arranged on alifting cylinder 26 (FIG. 2).

Two support arms 16 which, at their ends facing away from the axis ofrotation 15, are immovably connected with the housing panels 17, 18 ofthe infrared radiation modules 8, and are swivel-mounted on the axis ofrotation 15 of the deflection roller 7. The housing panels 19, 20 of theinfrared radiation modules 8 facing away from one another are alsoimmovably connected with two additional support arms 21, whose otherends are swivel-mounted on the axes of rotation 22 and 23 of the rollers6 and 9.

FIG. 3 shows the starting position of the cross-linking oven 13, whereinthe pipe 4 leaving the extruder 1 is led horizontally through thecross-linking oven 13.

In order to control the stretching processes S1 and S2 in accordancewith the previous embodiment, the cross-linking oven 13 must be adjustedfrom the starting position shown in FIG. 3 into an operating positionshown in FIG. 2. For this purpose, the lifting cylinder 26 must beextended, which is achieved by actuating a hand lever 24. As aconsequence, the deflection roller 7 moves to a raised position andtakes the infrared radiation modules 8 along with it above the supportarms 16, as a result of which the angles of the infrared radiationmodules 8 are adjusted synchronously with the height adjustment of thedeflection roller 7 through forced control by the pivoting support arms16 and 21, so that they adjust to the tilt resulting from the heightadjustment of the deflection roller.

Control of the extrusion line with regard to the stretching processes S1and S2 of the extruded pipe 4 takes place as in the previous embodiment.

The specification incorporates by reference the disclosure of Germanpriority document DE 10 2010 005 509.3 filed Jan. 23, 2010.

The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

1. A method of producing peroxide-cross-linked polyethylene pipes in anextrusion line that includes an extruder, a cross-linking oven and awithdrawal mechanism, wherein the cross-linking oven has a first heatingsection that extends from an inlet to an actuated deflection roller, anda second heating section that extends from the deflection roller to anoutlet, the method including the steps of: extruding a pipe via theextruder; subjecting the pipe to a stretching process; monitoring andregulating the stretching process such that stretching of the pipe takesplace primarily in the first heating section of the cross-linking oven,wherein stretching of the pipe in the second heating section iscontrolled to be close to zero; and withdrawing the pipe from thecross-linking oven via the withdrawal mechanism.
 2. A method accordingto claim 1, wherein the monitoring step in the first heating section ofthe cross-linking oven includes measuring the respective diameters ofthe pipe both prior to entry into, and after exit from, the firstheating section, and wherein the monitoring step in the second heatingsection of the cross-linking oven includes measuring the respectivediameters of the pipe both prior to entry into, and after exit from, thesecond heating section.
 3. A method according to claim 2, which includesthe further step of controlling a rotational speed of the deflectionroller such that an optimum stretching process of the pipe takes placein the first heating section.
 4. A method according to claim 3, whichincludes the further step of effecting a sequence control of the speedof the withdrawal mechanism as a constant function of the rotationalspeed of the deflection roller.
 5. A method according to claim 4, whichincludes the further step of linking a second control system with thesequence control of the withdrawal mechanism for reducing the speed ofthe withdrawal mechanism to such an extent that the stretching processof the pipe in the second heating section approaches close to zero. 6.An extrusion line for the production of peroxide cross-linkedpolyethylene pipes, comprising: a cross-linking oven having an actuateddeflection roller, wherein said cross-linking oven furthermore has afirst heating section that extends from an inlet to said deflectionroller and a second heating section that extends from said deflectionroller to an outlet; and means for adjusting a height of said deflectionroller and an angle of said first and second heating sections relativeto a horizontal plane.
 7. An extrusion line according to claim 6, whichcomprises further means for linking the adjustments of the height ofsaid deflection roller and the angle of said first and second heatingsections.